With a twin, the lower dipole needs to be at least a wavelength away from the building structure - you're going to need a bigger pole!

The "Super-J" would probably suit your site better, and would give you significant gain (if properly matched). It also has very little downward radiation, so there's little risk of local interference or overloading problems.

The big trick - whatever antenna you use - is to get the damn thing as high and as far as possible from the structure of the building!

With a twin, the lower dipole needs to be at least a wavelength away from the building structure - you're going to need a bigger pole!

The "Super-J" would probably suit your site better, and would give you significant gain (if properly matched). It also has very little downward radiation, so there's little risk of local interference or overloading problems.

The big trick - whatever antenna you use - is to get the damn thing as high and as far as possible from the structure of the building!

Thanks Mr 'H' .......
I've always wanted a bigger pole ever since I was a teenager

Anyway..... I'm trying stay as Inconspicuous as possible, but I also thought I need to go higher...

How about like this... giving me an extra 1.3M

2 Bay Dipole-3 + Extra 1.3M.jpg

The "Super-J" you mean this one ? .... or another ?
on eBay >>> FM band 88 to 108 mhz jpole antenna

JPole.jpg

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''Radio Brian'' - One of the Most Unique English Speaking Radio Stations in the Canaries.

Some web sites suggest the Super-J has a full 3 dB gain over the plain J-Pole which is a pretentious claim obvious to anyone with a copy of the ARRL Antenna Book and other references. Indeed, the Super-J will never approach the gain advantage of a properly separated two element collinear antenna, but we can get closer to this goal.

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''Radio Brian'' - One of the Most Unique English Speaking Radio Stations in the Canaries.

Getting your twin stack that bit higher will help, but a good vertical colinear would be even better.....

The Super-J is a reasonably easy aerial to build, but the centre matching section always weakens the aerial, which might pose a problem out at sea as you are.

You could consider a ⅝-wave or a ⅞-wave according to high up your site is. These can be made to look (almost) like base-loaded CB aerials, so they're really good for keeping a low profile. Look at the radiation angle you want, and decide on the antenna.

Each will require ¼-wave ground planes (minimum 3, 4 is better, but 6 is great!) angled downwards (the angle affects the match somewhat). This type of aerial also gives you other advantages - low radiation downwards into the building, low field strength in the immediate vicinity (confuses people trying to track it down) and it is an unbalanced aerial, so no balun is required. There are plenty of on-line calculators that will work out the dimensions for you and the number of turns on the matching coil.

These groundplane types are commonly used for amateur repeaters and commercial PMR base stations, where the operators want fully omnidirectional coverage, useful antenna gain, and a robust, easily constructed, aerial.

Build one, they're easy to build from standard parts from a plumbing shop, easy to tune and you'll get some satisfaction out of it. You'll have to tune the eBay one anyway. I'm not keen on those eBay ones - your coax will fill with water.